Solid state capacitor structures are well known in the art and employed for a variety of applications such as sensors, accelerometers, actuators and the like. The vast majority of well known capacitor structures generally include two conductive plates and measure only the capacitance therebetween. There are relatively few integrated capacitor structures capable of measuring differential or relative capacitances.
U.S. Pat. No. 4,736,629 entitled "Micro-Miniature Accelerometer" issued to John Cole on Apr. 12, 1988 discloses an accelerometer structure that may measure two distinct capacitances to determine differential or relative capacitance. The structure disclosed by the '629 patent comprises a substrate having a cavity in which a mounting system is disposed. A torsion bar or beam connects the mounting system to a moveable plate so that the plate is positioned above the substrate and can rotate about a flexure axis that is above and substantially parallel to the substrate. The flexure axis divides the sensing element into two separate sections. Basically, the structure is similar to a see-saw.
A first electrode is mounted by the substrate adjacent the first section of the sensing element and forms a first capacitor while a second electrode is mounted by the substrate adjacent to the second section of the sensing element and forms a second capacitor. A detector for measuring the relative capacitance of the first and second capacitors is provided.
Although the structure taught by the '629 patent is adequate for many applications, it has substantial drawbacks that make its use less than optimum. To have adequate sensitivity, the cantilevered see-saw structure requires distance between the first and second sections of the sensing element. This requires relatively long arms and a large amount of area. Additionally, the torsion bar structure must be asymmetric so that the center of mass is offset and differential capacitance may be measured. Further, the structure is relatively difficult and expensive to fabricate.
Accordingly, it would be highly desirable to have a differential capacitor structure that is small in size, symmetric and that may be manufactured relatively easily and inexpensively.